the materials are not deleted and created any more after closing the material selection dialog => no need to take care of material deletion

...and simplify code

Add and use dedicated methods. This will ease SessionModel migration

Custom editors are now only used from the Fit-Parameter-tree. In this tree, only doubles and Combos exist, and doubles use only scientific spin boxes.
All other custom editors are obsolete

...and remove obsolete code.

The "model tech view" is only kept for further migration (to understand the SessionModel). However, it is not allowed anymore to edit the model in this view.

-> SessionModelDelegate
-> FitParameterDelegate